How do Trojan viruses infect your computer? Where does spam come from? How do hackers gain control over your computer?

Often times, the answer is via botnet.

A botnet is nothing more than a small program that people install (usually without realizing it) that "phones home" to another computer and carries out whatever instructions it's given.

On it's own, a single infected computer isn't much to write home about. But when you're talking about 100 or 1,000 (and often times even more) computers, a hacker has a lot of computing power at his disposal.

One of the most popular means of gaining access to a computer is to hide a Trojan in an email attachment that joins the botnet upon installation. This happens without the user's knowledge or consent. From here, the botnet owner can deploy commands to his collection of compromised computers.

One of the most widely-used botnets is called Zeus.

You can customize your own build using free software.

Using readily-available software that's nothing more than a search away, people who mean you harm can get a customized Zeus botnet up and running in minutes. At this point, they distribute it, most likely via email.

They'll employ a little bit of human engineering to get you to install a program that connects back to a server that they control. At this point, they are able to send commands to your computer.

There are a few communities online where people trade botnet tricks and tips.

The Botting Network is one of the largest of these. People who run their own botnets (whether for legitimate or illegitimate purposes) will come together to trade tips and thoughts on maintaining a botnet.